In recent years, with remarkable progress of inverter circuits and switching devices in performance, further improvement in high frequency characteristics and reliability is required for a field effect transistor, hereinafter referred to as FET.
Therefore, for instance, a multi-finger FET is used with a gate circuit line fabricated in parallel to a channel region, being connected with a plurality of gate fingers fabricated so as to cross the channel region, and a source circuit line or drain circuit line for connecting a bonding pad with a source electrode or a drain electrode fabricated on the channel region. The gate circuit line intersects with the source circuit line or the drain circuit line, however to insulate them, a passivation layer of SiN or the like is fabricated on the gate circuit line. However, by thus forming the circuit lines directly on the passivation layer of SiN or the like having high dielectric constant, stray capacitance is generated. Particularly, in a high-frequency region, the stray capacitance is not negligible. Accordingly, to reduce the stray capacitance, there has been used an air-bridge structure, in which an upper layer circuit line is fabricated over an air gap (see patent documents 1).
In this type of air-bridge structure, source/drain electrodes are composed of a metal layer of, for example, Pt/AuGe layers, which have ohmic contacts with the channel region, and a metal layer of, for example, Au/Pt/Ti layers piled on the metal layer having ohmic contacts. On the whole surface of the metal layers, regions for source/drain bonding pads and a connecting region between them (an air-bridge), for example, a single plated layer of Au is fabricated. Au composing the air-bridge has a higher thermal expansion coefficient than GaAs substrate. Accordingly, temperature changes, from a plating temperature (e.g. 60° C.) to an energizing temperature (e.g. 225° C. which is the burn-in temperature) or to a non-energizing temperature (e.g. 25° C. which is the room temperature), and hence thermal expansion or thermal contraction causes at the air-bridge. Such thermal expansion or thermal contraction generates a large internal stress such as compressive stress or tensile stress in the channel region. Thus, the internal stress causes problems of degradation of output power or low reliability.
[Patent Document 1]
Japanese Patent Application laid-Open No. 9-8064 (e.g. FIG. 1)